Measurement instrument and method

ABSTRACT

A measurement instrument having a processor, a first sensor and a second sensor. The processor is adapted to output a measurement signal embodying a measurement of a physical quantity. The first sensor and second sensor are connected to the processor and are operable to generate respectively first and second measurements of the physical quantity. The processor defines a first measurement range within which the measurement signal is dependent on the first measurement and not the second measurement. The processor defines a second measurement range within which the measurement signal is dependent on the second measurement and not the first measurement. The first and second ranges meet at a predetermined transition. The first and second measurements are different at the transition and the measurement embodied in the measurement signal crosses the transition without an abrupt change.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and claims the benefit of U.S.patent application Ser. No. 13/487,018 filed on Jun. 1, 2012 (now U.S.Pat. No. 8,589,107) which is a continuation of Ser. No. 11/739,986 filedon Apr. 25, 2007 (now U.S. Pat. No. 8,195,418), the disclosures of whichare incorporated herein by reference in their entireties.

BACKGROUND

1. Field of the Exemplary Embodiments

The exemplary embodiments disclosed herein relate to measurementinstruments and, more particularly, to measurement instruments havingmultiple sensors.

2. Brief Description of Related Developments

Many different types of sensors have been used to measure variousphysical quantities, for example pressure or density of a gas. Asdifferent types of sensors may have different operating ranges, it hasbeen desired to combine different types of sensors into a singlepressure instrument, with an extended operating range. For example, asthe pressure of a gas is pumped down to vacuum, the output of theinstrument may first correspond to a reading from one of the sensors.Then, when the output reaches a threshold value, the output may beswitched to correspond to a reading from another sensor having betteraccuracy at the lower pressures. While this type of arrangement has anadvantage in extending the pressure or density range over which theinstrument can operate with suitable accuracy, there are also drawbacks.Most notably, a problem may arise in switching between the two sensors.If the two sensors do not produce identical readings at the thresholdvalue, there may be an abrupt change in the output of the instrumentwhen the instrument switches between sensors. Even if the difference inreadings between the two sensors is small, the abrupt change can causeundesirable hysteresis effects. For example, problems may arise if theoutput is used as part of a feedback loop designed to control pressure.The difficulties may be more pronounced if a derivative of the output isused as a feedback signal in a feedback loop, because the derivativewill be very high at the transition threshold. Therefore it may bedesired to provide a pressure instrument that combine readings from twoor more sensors and allow for smooth transitioning between the readings.

SUMMARY

In one exemplary embodiment, a measurement instrument having aprocessor, a first sensor and a second sensor is provided. The processoris adapted to output a measurement signal embodying a measurement of aphysical quantity. The first sensor and second sensor are connected tothe processor and are operable to generate respectively first and secondmeasurements of the physical quantity. The processor defines a firstmeasurement range within which the measurement signal is dependent onthe first measurement and not the second measurement. The processordefines a second measurement range within which the measurement signalis dependent on the second measurement and not the first measurement.The first and second ranges meet at a predetermined transition. Thefirst and second measurements are different at the transition and themeasurement embodied in the measurement signal crosses the transitionwithout an abrupt change.

In another exemplary embodiment, a pressure gauge comprises a pressureindicator. A first pressure sensor is connected to the pressureindicator and is operable to generate a first pressure reading. A secondpressure sensor is connected to the pressure indicator and is operableto generate a second pressure reading. The pressure indicator isconfigured to indicate pressure responsively to the first pressurereading, without being responsive to the second pressure reading, whenthe first pressure reading is above a predetermined pressure thresholdand falling. The pressure indicator is configured to indicate pressureresponsively to the second pressure reading, without being responsive tothe first pressure reading, when the first pressure reading is below thepredetermined pressure threshold and falling. The pressure indicator isconfigured to indicate pressure as a continuous function over anindicated pressure range that includes the threshold pressure, with thesecond pressure reading being different from the first pressure readingwhen the first pressure reading is substantially equal to thepredetermined pressure threshold.

In still another exemplary embodiment, a pressure gauge comprises apressure indicator. A first pressure sensor is connected to the pressureindicator and operable to generate a first pressure reading. A secondpressure sensor is connected to the pressure indicator and is operableto generate a second pressure reading. The pressure indicator isconfigured to indicate pressure responsively to the first pressurereading, without being responsive to the second pressure reading, whenthe first pressure reading is below a predetermined threshold pressureand rising. The pressure indicator is configured to indicate pressureresponsively to the second pressure reading, without being responsive tothe first pressure reading, when the first pressure reading is above thepredetermined threshold and rising. The pressure indicator is configuredto indicate pressure as a continuous function over an indicated pressurerange that includes the threshold pressure, with the second pressurereading being different from the first pressure reading when the firstpressure reading is substantially equal to the predetermined pressurethreshold.

In yet another exemplary embodiment, a method comprises producing afirst reading of a quantifiable physical property with a first sensor.The method further comprises producing a second reading of thequantifiable physical property with a second sensor. The method stillfurther comprises indicating a magnitude of the physical propertyresponsively to only the first reading over a first range of thephysical quantity. The method yet further comprises indicating amagnitude of the physical property responsively to only the secondreading over a second range of the physical quantity that adjoins thefirst range at a transition magnitude of the physical property. Themethod further comprises adjusting the indicated magnitude of thephysical property in at least one of the first or second regions so thatthe indicated magnitude does not change abruptly in transitioningbetween the first and second ranges, the first and second readings beingdifferent at the transition magnitude.

BRIEF DESCRIPTION OF THE DRAWINGS

The exemplary embodiments are explained in the following description,taken in connection with the accompanying drawings, wherein:

FIG. 1 is a top schematic view of a substrate processing apparatusincorporating an exemplary embodiment of a measurement instrument;

FIG. 2 is a schematic illustration of the exemplary embodimentmeasurement instrument of FIG. 1;

FIG. 3 is a system diagram illustrating connectivity between variousparts of the processing apparatus of FIG. 1;

FIG. 4 a is a signal diagram illustrating sensor signals in theexemplary embodiment;

FIG. 4 b is another signal diagram illustrating sensor signals in theexemplary embodiment;

FIG. 5 is a flow chart corresponding to the exemplary embodiment;

FIG. 6 is another signal diagram corresponding to the exemplaryembodiment.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENT(s)

FIG. 1 illustrates an exemplary substrate processing apparatus 100. Theapparatus may have an exemplary embodiment of a pressure measurementinstrument 400, as further described below. The substrate processingapparatus is described as an example application of the pressuremeasurement instrument 400. It should be understood that the pressuremeasurement instrument may be used in any suitable application, such asto measure a pressure or gas density within any suitable pressure orvacuum chamber, or may operate independently of any pressure or vacuumchamber. Although the exemplary embodiments will be described withreference to the embodiments shown in the drawings and described below,it should be understood that these aspects could be embodied in manyalternate forms of embodiments. In addition, any suitable size, shape,or type of elements or materials could be used. In FIG. 1, the apparatus100 may, for convenience, be described as having a front end 200 and aback end 300. The front end 200 may have a frame 230 and may comprise asubstrate transport apparatus 210 operating within, for example, acontrolled environment 105. Transport apparatus 210 may have an arm 215and a drive mechanism 220 that is operable to move the arm. The arm 215may have an end effector 250 for supporting a substrate.

The front end 200 of the example apparatus 100 may also include loadports 120,125 (two are shown for example purposes). The load portsprovide an interface with substrate cassettes 130. Each substratecassette is adapted to hold several substrates, and may hold them withina sealed enclosure. The load ports 120,125 removably hold the cassettes130 and may include mechanisms (not shown) to open cassette doors,allowing access to the substrates located in the cassettes from within acontrolled environment 105 of the front end 200. The controlledenvironment 105 may be contained by a housing, and may be connected to apressure gauge 470 for measuring pressure of the controlled environment105. Opposite the transport apparatus 210 from the load ports 120,125are load locks 135,140. Load locks 135,140 connect the front end 200 tothe back end 300. Each load lock 135, 140 has a valve 405, 410connecting it to the controlled environment 105 of the front end 200 andanother valve 415, 420 connecting it to the isolated environment 310contained in the transport chamber 305 of the back end 300. The isolatedenvironment 310 of the transport chamber 305 may, for example, be avacuum, inert gas such as nitrogen, or other fluid. The controlledenvironment 105 of the front end 200 may be clean air at atmosphericpressure, maintained with very low levels of particulate contaminants.Thus, the load locks 135,140 allow passage of substrates between thefront end 200 and the back end 300 while maintaining isolation of thetwo environments 105, 310. In FIG. 1, load lock 140 is shown connectedto both an inlet valve 480 for pressurizing the load lock, and a vacuumpump 465 for depressurizing the load lock.

In the example apparatus 100, back end 300 comprises a frame 315defining a transport chamber 305. As noted above, the transport chamber305 may hold an isolated environment 310, such as a vacuum. A pressuregauge 475 may be connected to the transport chamber 305 for readingpressure of the isolated environment 310. A substrate transportapparatus 320 may be located within the transport chamber 305. Transportapparatus 320 may comprise a drive mechanism 325 connected to the frame315 and a pair of opposing arms 335,340 connected to an end effector365.

In the example apparatus 100 of FIG. 1, several processing modules 370are located on the periphery of the transport chamber 305. Theprocessing modules 370 may operate on the substrates through variousdeposition, etching, polishing, cleaning or other types of processes toform electrical circuitry or other desired structure on the substrates,or to perform metrological or other functions. The processing modules370 are connected to the chamber 305 to allow substrates to be passedfrom the transport chamber to the processing modules and vice versa. Theprocessing modules, in the exemplary embodiment, may be isolatable fromthe transport chamber, for example via slot valves or other suitabledoors, hence one or more of the processing modules may be capable ofholding an atmosphere, such as an inert gas (e.g. nitrogen, argon), thatis isolated from the atmosphere or vacuum in the transport chamber 305.One or more of the processing modules may be subjected to varyingpressures, such as an operating pressure during workpiece processing(e.g. low vacuum) a loading pressure (e.g. high vacuum duringloading/unloading of workpieces) and access pressure (e.g. ambientatmospheric for maintenance of the module). Similarly the transportchamber 305 may also be subjected to different pressures. In theexemplary embodiment one or more of the processing modules may have apressure gage or measurement instrument 400 capable of measuring thepressure in the processing modules as will be described further below.

Load lock 140 may be connected to a pressure measurement instrument 400as well. Load lock 135 may also be connected to a similar pressuremeasurement instrument. The pressure measurement instrument 400 maymeasure an absolute pressure of an atmosphere 450 within a desiredsection(s) of the apparatus 100 such as the load lock 140, transportchamber 305 or processing modules. In alternate embodiments, themeasurement instrument may measure a relative or differential pressure,or both. In other embodiments, instrument 400 may measure a gas densityor other physical characteristic that may serve as a proxy for pressuremeasurement. In still other embodiments, a measurement instrument maymeasure any suitable physical characteristics.

FIG. 2 is a schematic illustration of the measurement instrument 400 inaccordance with an exemplary embodiment. In the exemplary embodimentshown, measurement instrument 400 may be an integrated instrument,housed for example in a casing or housing 400C. Accordingly, in theexemplary embodiment, the measurement instrument 400 may be modularlyinstalled to any one or more of the desired sections of the processingapparatus 100 such as the transport chamber, processing modules orloadlocks for example. In alternate embodiments, the measurementinstrument may be installed to measure desired physical characteristicsof an atmosphere in any desired apparatus. In other alternateembodiments, the measurement instrument as will be described furtherbelow may be integrated within the apparatus or tool holding theatmosphere measured by the instrument. As seen in FIG. 2, in theexemplary embodiment, the casing 400C may be configured to define apressure envelope 425 in communication with atmosphere 450 that is heldwithin the measured section of apparatus 100, (e.g.) the load lock 140or process module(s) 370). Hence, pressure within the pressure envelope425 is substantially the same as pressure within the load lock 140(loadlock 140 as referenced below is merely a representative section ofapparatus 100). In the exemplary embodiment the pressure envelope isdepicted as being located within the housing for example purposes only(a port for example may be provided between the interior of theapparatus with atmosphere 450 and the pressure envelope 425 forcommunication). In alternate embodiments, the pressure envelope of themeasurement instrument may be located as desired including exterior tothe instrument housing. In the exemplary embodiment shown, threepressure sensors may be connected to the pressure envelope and operableto measure pressure within the envelope. In alternate embodiments moreor fewer pressure sensors may be connected to sense the pressure in thepressure envelope. Other alternate embodiments may or may not have apressure envelope, and may have any suitable number of sensors. In theexemplary embodiment, pressure sensor 435 may comprise a piezo-resistivediaphragm (PRD). One side of the diaphragm may be connected to thepressure envelope 425. Another side of the diaphragm may be sealed offat high vacuum so that the PRD is an absolute diaphragm sensor. The PRDsensor 435 may be operable to detect an absolute pressure of thepressure envelope 425 in pressure envelope 425. For example, a change inpressure may cause a stress or change in stress or strain within thepiezo resistive diaphragm that affects the resistivity of the diaphragm.The sensor 435 may hence, sense a pressure or change in the pressure inenvelope 425 by measuring a change in electrical resistance in thediaphragm. Sensor 440 may also be connected to the pressure envelope425. Sensor 440 may be, for example, a heat loss sensor. Heat losssensor 440 may be operable to sense an absolute pressure within thepressure envelope 425 by sensing heat loss from an electrical conductor.The heat loss may correspond to the pressure within the pressureenvelope 425, and hence for example within the load lock 140. Sensor 445may also be connected to the pressure envelope 425. Sensor 445 may be,for example, an ionization sensor. The ionization sensor 445 may emitelectrons that collide with gas particles in the measured gas. Thecollisions may create ions that conduct an electrical current. Theamount of current may correspond to gas density and gas pressure withinthe pressure envelope 425, and hence may correspond to pressure withinthe load lock 140.

In the exemplary embodiment, each of the sensors 435, 440, 445 may havea different operating pressure range. For example, a high vacuum (e.g.10̂−10 Torr to 10̂−2 Torr) may be most accurately measured by theionization gauge (IG) 445. A medium vacuum (e.g. 10̂−3 to 1000 Torr) maybe most accurately measured by the heat loss (HL) sensor. The PRD sensor435 may be used for measuring low vacuum to atmospheric pressure asillustrated substantially in FIG. 2. Instrument 400 may include a signalprocessor 455 that is communicably connected to the respective sensors435, 440, 445. In general, the signal processor is capable of receivingthe sensor signals and converting them to output a desired measurementsignal. As may be realized, the measurement instrument 400 may be usedto measure transient and steady state pressure, within one or moredesired chambers or modules of the processing apparatus 100, that mayrange from atmospheric to high vacuum (e.g. 13 decades). The processor455 is capable of using the signals from the sensors 435, 440, 445 togenerate the output measurement signal indicating the measured pressure(or other measured characteristic) in the range from about 10̂−10 Torr toabout 1000 Torr as will be described below.

FIG. 3 illustrates schematically various connections of the measurementinstrument 400 to the substrate processing apparatus 100. As notedpreviously, the measurement instrument in the exemplary embodiment (seealso FIG. 2) may be an integrated package that may be modularlyinstalled or removed from the apparatus 100. The measurement instrumentmodule may be mechanically connected to the desired sections of theapparatus 100 (e.g. transport chamber 325, process modules 370, loadlock(s) 135, 140) so that the pressure envelope 425 of the instrumentmodule is in communication with the desired atmosphere 450 within theapparatus. For example, the instrument housing 400C may be attached tothe frame of apparatus 100 by suitable fasteners (e.g. screws, quickrelease clamps, etc.). The instrument module 400 may also beelectrically connected, for example via a wired (e.g. USB) or wirelesscoupling 458, to the apparatus 100 for power and communication. Thesignal processor 455 may be connected via coupling 458 to a controlsystem 460 of the apparatus 100. In the exemplary embodiment, the signalprocessor may be operable to generate an output measurement signal,using multiple readings from the sensors 435, 440, and 445, that iscommunicated to the control system 460. In the exemplary embodiment, thecontrol system 460 may also be connected to other devices of theprocessing apparatus 100, such as the substrate transport apparatus 210and 320. The control system may also be connected, for example, to theload ports 135, 140 processing modules 370, any other suitable devicesof the apparatus 100 and any devices external to the apparatus 100. Thecontrol system 460 may be connected to a vacuum pump or system 465 forregulating pressure within the transport chamber 305, modules and loadlock(s) 140 of the apparatus. In the exemplary embodiment, the controlsystem may also be connected to an absolute pressure sensor 470 formonitoring pressure within the controlled atmosphere 105 of the frontend 200. The control system 460 may be connected to a vent valve 480 forventing load lock 140, and may be connected to loading valves 410, 420of the load locks and process modules. Thus, the control system 460 maybe operable to coordinate operations of the apparatus 100, such astransport of substrates through the load locks 135, 140 and otherprocess modules. Processor 455, of the instrument module, and/or controlsystem 460 may also be directly or remotely connected to a suitable I/Odevice 510 (see FIG. 3) that is capable of inputting information tocontrol system 460 and processor 455, and also capable of displayinginformation, such as the indicated pressure measurement (or othermeasured characteristic) output from the processor 455.

In one example operation of the processing apparatus 100, a substratemay be removed by transport apparatus 210 from a substrate cassette 130docked at load port 125. Pressure sensor 470 may measure pressure withinthe controlled environment 105 and communicate the measured pressure tocontrol system 460. The control system 460 may also identify and monitorthe operating status of the desired sections of apparatus 100. Forexample control system 460 may determine or confirm whether the desiredpressure or vacuum condition is present in the transport chamber 305 andprocess modules using measurement instrument 400. As may be realized, inthe event conditions do not conform to desired protocol, the controlsystem may generate a fault signal and may for example initiate(automatically or with operator input) remedial procedures. By way ofexample, if the control system registers from the output of measurementinstrument 400, that the transport chamber is not at a desired vacuumcondition, the control system may activate vacuum pumps to establishdesired vacuum. Upon receiving indication from measurement instrument400 that desired vacuum has been established in the transport chamber305, the control system may automatically deactivate the vacuum pump.Generation or maintenance of desired atmospheric condition may besimilarly effected in any desired section or module of the apparatus100. The control system 460 may also determine the status of and controlthe operation of load lock(s) 135, 140 using the measurement instrumentconnected to the load ports. By way of example, the measurementinstrument 400 may measure pressure within the load lock 140 using thepressure sensors 435, 440, and 445, and send an indication (from thesignal processor 455) of the measurement to the control system 460. Thecontrol system may compare pressure measurement indications between loadlock 140 and control environment, and operate the vacuum pump 465 tocontrol the pressure within the load lock 140 so as to equalize pressurebetween the load lock atmosphere 450 and controlled environment 105.Both valves 410, 420 of the load lock 140 may be sealed as the pressureis equalized. In equalizing the pressure, the control system may use theunified output from the signal processor 455 to provide a feedbacksignal for controlling the vacuum pump 465. As noted before, and as willalso be described in greater detail below, the pressure variance in theload lock 140 (e.g. between matching atmospheres with the transportchamber and the environmental front end), or any other desired chamberof the apparatus, may be such (e.g. decades) that the measurement rangeof each sensor 435, 440, 445 may not be sufficient to accurately measurepressure across the whole pressure variance. Thus the pressureindication from the measurement instrument measuring the pressure in theload lock may be based on two or more of the sensors 435, 440, 445. Forexample the pressure indicated by the instrument when the load lock, orany other chamber/module, is at high vacuum may be based on the IGsensor 445, at low vacuum the indication may be based on the HL sensor440, at atmospheric pressure the indication may be based on the PRDsensor 435. As may be realized from FIGS. 1 and 3, valve 410 between theload lock 140 and controlled environment 105 may be opened by controlsystem 460 upon receiving a suitable pressure indication from theprocessor 455 allowing the control system to determine that pressuresacross valve 400 are balanced. The pressure indication from sensor mayalso be displayed on I/O device 510 (see FIG. 3). Because pressurebetween the load lock atmosphere 450 and controlled environment 105 issubstantially equal, there may be no rush of gas upon opening of thevalve 410, which could cause dispersal of contaminants onto thesubstrate. The control system 460 may then direct the substratetransport apparatus 210 to transfer substrate(s) between the load lock140 and front end. Upon completion of substrate transfer, the valve 410may be closed, leaving the load lock atmosphere sealed off from thecontrolled environment 105 of the front end, as well as from theisolated environment 310 of the back end 300. The control system mayequalize pressure between the load lock atmosphere 450 and the isolatedenvironment 310 by pumping down the pressure within the load lock usingthe vacuum pump 465. The measurement instrument 400 may provide afeedback signal to the control system 460, which may be used incontrolling the vacuum pump 465. The control system may also monitorpressure within the isolated environment 310 using pressure sensor 475.When the control system has determined that pressure is substantiallybalanced between the load lock atmosphere 450 and isolated environment310 of the transport chamber 305, the control system may open the valvebetween the load lock 140 and the transport chamber 305 for substratetransfer between the load lock and transport chamber. This process maybe repeated as desired for loading and unloading substrates between thefront and back ends of the apparatus 100. Similarly, the measurementinstruments 400, mounted on one or more of the process modules and thetransport chamber, may facilitate loading/unloading of substratesbetween transport chamber and process module(s) isolated from thetransport chamber during processing. As may be realized, the exampleoperations described above merely serve to illustrate applications ofthe exemplary embodiment measurement instrument 400. The measurementinstrument 400 may be used in any suitable application, such as anysuitable application wherein a pressure or density of a gas is measured.In alternate embodiments, the measurement instrument may measure anysuitable physical quantity or quantities.

As noted before, in the exemplary embodiment, the pressure instrument400 may be arranged to measure pressures throughout the full pressurevariance (e.g. 13 decades; from 10⁻¹⁰ to 10⁻³ Torr) in the desiredchamber/module using the three sensors 435, 440, 445 (though inalternate embodiments, depending on the extent of the pressure change inthe chamber, more or fewer sensors may be used). As also noted before,in the exemplary embodiment, each sensor 435, 440, 445 may have adifferent pressure range, the different pressure ranges of the threesensors being used together to provide instrument 400 with the desiredoverall measurement range. The processor 455 (see FIGS. 2-3) may beprogrammed, in the exemplary embodiment, so that its measured pressureoutput (i.e. indicated pressure from the instrument) is based on thebest data from the sensors 435, 440, 445. Accordingly, in the exemplaryembodiment, at a given pressure, the processor 445 programming uses themeasurement data from the sensor with the greatest accuracy (i.e. bestdata) for the given pressure as will be described below. In theexemplary embodiment, the pressure range of each sensor 435, 440, 445,or in other words the range of pressure where each sensor has thegreatest accuracy relative to the other sensors of the instrument may beprogrammed in processor 455. By way of example, the processor 455 (or asuitable memory location accessible by processor 455) may haveprogramming 456 such as a suitable algorithm or lookup table to identifyand select the pressure range for each sensor. For example, theprocessor may directly or indirectly use the calibration line relatingindicated pressure (e.g. output signal as a function of true pressure)from each sensor with true pressure to establish the gage pressurerange. The calibration information for each particular sensor 435, 440,445 may be downloaded, for example over a global or local network(s), orotherwise entered in any other suitable manner into the processor 455.In alternate embodiments, the sensor calibration information may bestored at a remote location (for example a PC at the instrumentmanufacturer facility) and accessed as desired by the processor via asuitable bi-directional communication path. In other alternateembodiments, the calibration information may be generated by theprocessor 455, with the instrument mounted in place and using knownsensor calibration techniques. FIG. 6 graphically illustratesrepresentative calibration lines (e.g. gain) PS1, PS2 for two sensors435, 440, 445. As may be realized, the calibration plots or lines forsome of the different types of sensors (e.g. IG sensor 445 or HL sensor440) may vary for different gas species (e.g. controlled air, N₂,argon). In the exemplary embodiment, the programming of processor 455may be arranged to effect selection of the sensor calibrationinformation corresponding to the gas species expected to be measured inthe chamber. For example, the calibration lines of each sensor fordifferent gas species may be stored in program 456 of processor 455. Theprocessor program 456 may correlate the respective calibration lines tothe corresponding gas species for each sensor, and select theappropriate calibration line upon receiving an input, such as from theI/O device 510 or any other desired means, identifying the gas speciesin the chamber the pressure of which is to be measured with instrument400. In alternate embodiments, the processor 455 may read or downloadthe appropriate calibration line from the remote location afterreceiving the input identifying the gas species in the chamber.

As may also be realized, for sensors of a given type (e.g. IG, HL orPRD), the calibration lines may also vary from sensor to sensor. Thecalibration lines PS1, PS1′ and PS2, PS2′ shown in FIG. 6 graphicallyillustrate the variance that may exist in calibration lines or gain fordifferent sensors of the same type. As noted before the plots shown inFIG. 6 are merely exemplary. In the example shown, lines PS2, PS2′ maycorrespond to different PRD sensors and lines PS1, PS1′ may correspondto different HL sensors. Lines PS2, PS1 may correspond to PRD and HLsensors 435, 440 of instrument 400 respectively. The characteristics ofthe lines PS1, PS1′, PS2, PS2′ are merely exemplary and in alternateembodiments the calibration lines of the sensors may have any otherdesired characteristics. The calibration lines PS2, PS2′ for thedifferent PRD sensor(s) are shown for example only as having arelatively higher gain than the calibration lines PS1, PS1′ marking theperformance of the different HL sensors. In alternate embodiments, therelative gains for the different types of sensors may be different. Thecalibration lines for the IG sensor (not shown) may have generallysimilar characteristics to the calibration lines shown in FIG. 6, andthe relationship between calibration lines of the IG sensor (such assensor 445) and HL sensor (such as sensor 440) may be for examplegenerally similar to that shown for the calibration lines PS2, PS2′,PS1, PS1′ in FIG. 6. In one exemplary embodiment, the processor 455 maybe programmed to calculate or otherwise identify for example, from thecalibration line PS1, PS1′, PS2, PS2′ of the corresponding sensors 435,440, 445 for the given gas species the pressure (or other measuredcharacteristic) range where each sensor provides the best data. By wayof example, the calibration lines of the specific sensors 435, 440, 445(of instrument 400) for the gas species registered by the processor (asnoted before) may be compared to desired thresholds to establish thebest data pressure ranges (or what shall be referred to hereafter as thepressure range) of each sensor 435, 440, 445 for the given gas species.In alternate exemplary embodiments, the pressure ranges of the sensorsmay be entered into or downloaded by the processor from a remotelocation. As may be realized from the above, the pressure rangeregistered in the processor may be sensor specific and species specific.By way of example, the processor programming 456 may include information(such as an algorithm or table) that defines range information for eachsensor 435, 440, 445 corresponding to the various gas species. As notedbefore, with the pressure range of each sensor 435, 440, 445 registeredby the processor, the processor 455 in the exemplary embodiment may usethe pressure reading from the sensor(s) with the appropriate pressurerange as the sensors measure the pressure change in the chamber.Moreover, as may be realized in the exemplary embodiment the processor455 may transition between sensors when the pressure(s) measured exceedthe pressure range of the given sensor.

In order to prevent gaps in the instrument 400 measurement range, thepressure ranges for the sensors 435, 440, 445 may be established toprovide desired overlap. Thus for example the pressure range for PRDsensor 435 may overlap the pressure range of the HL sensor 440, which inturn may overlap the pressure range of the IG sensor 445. Arepresentative overlap region between two sensors 435, 440, 445 isgraphically illustrated in FIG. 4 a that shows lines 501, 502 depictingsensor readings for the sensors within the corresponding sensor pressureranges. As may be realized, the sensor performance lines 501, 502 inFIG. 4 a (i.e. relating sensor readings to true pressure for therespective sensors) are substantially similar to portion(s) of thesensor calibration lines PS1, PS2 shown in FIG. 6. Further, in theexemplary embodiment lines 501, 502 may also represent the indicatedpressure (P_(ind)) from the processor (relative to true pressure).Hence, and as will be described in greater detail below, in theexemplary embodiment, the indicated pressure from processor 455 may be afunction of the present value (i.e. pressure reading) of but one sensor435, 440, 445 even in the overlap region. Accordingly, as may berealized from FIG. 4 a, the amount of overlap can be minimized and bemuch smaller than range overlap desired with conventional systems. Forexample, and as will also be described further below, the overlap may beso that when one sensor 435, 440, 445 is at the end of its range, thereading from the other sensor is valid hence allowing the processor toswitch between sensors. However, as noted before the sensor pressurerange, and hence the end points of the range may vary between gages (ofthe same type) as well as between gas species. In the exemplaryembodiment, the processor may be arranged to select or allow selectionof the end point(s) of the pressure range for the sensors, and henceselection of the switch pressure (P_(ind)) where the processor switchesbetween sensors, for example so that the pressure indication from theinstrument 400 may be based on best data using the current pressurereading (value) from but one sensor 435, 440, 445. In alternateembodiments, the switch pressure (Psw), where the pressure indicationswitches sensors, may be entered into the processor by the operator, forexample via the I/O device 510 (see FIG. 3), or maybe downloaded by theprocessor from a remote location.

Referring again to FIG. 4 a, which illustrates, as noted before, anexample of how pressure readings from two sensors may vary with respectto a true pressure. In FIG. 4 a, line 501 may represent, for example, apressure reading from ionization sensor 445. Line 502 may, for example,represent a pressure reading from heat loss sensor 440. As describedabove heat loss sensor 440 and ionization sensor 445 may have differentoperating ranges. Therefore it may be desired to use a pressure readingfrom ionization sensor 445 at pressures below a switching pressureP_(SW) as a basis for an indicated pressure from the instrument 400, anduse a pressure reading from heat loss sensor 440 as a basis for theindicated pressure at pressures above the threshold pressure P_(SW). Asseen best in FIG. 4 a, there is a variance between the readings from thetwo sensors (see also FIG. 6). Thus, as the sensors 440, 445 may notproduce identical pressure readings at the threshold pressure P_(SW),switching between the two sensor readings at the switching pressurewould result in an abrupt jump or discontinuity in the indicatedpressure from the instrument. The presence of discontinuities in a datastream that may be subjected to some kind of processing are highlyundesired. For example, differentiating an indicated pressure at or nearthe threshold pressure P_(SW) might then result in a high magnitudevalue for rate of change in pressure, which could introduce hysteresisin a pressure control feedback system or generate faults in theprocessing system. For example, when monitoring an environment ofdecreasing pressure, a reading from heat loss sensor 440 may first beused to determine the indicated pressure P_(IND). This would follow line502. When a reading from heat loss sensor 440 reaches the thresholdpressure P_(SW), a reading from the ionization sensor 445 may then beused to determine the indicated pressure P_(IND). This would follow line501. However, as exemplified in FIG. 4 a, when heat loss sensor 440 isreading the pressure as equal to P_(SW), the true pressure would beP_(C) and ionization sensor 445 would read a pressure of P_(B). As P_(B)does not equal P_(SW), a sharp change in the indicated pressure P_(IND)would occur. This sharp change in pressure would not reflect the changein true physical pressure, but would merely be an artifact introduced bythe measurement instrument. A similar phenomenon may occur in a risingpressure environment, where P_(IND) first follows the line 501 readingsof sensor 445, and then shifts to follow the line 502 readings of sensor440. When a reading from sensor 445 reaches the threshold pressureP_(SW), true pressure would be P_(D) and the pressure reading of sensor440 would be P_(A). As P_(A) is a different value than P_(SW), merelyswitching from one sensor reading to another may create a sharp jump ora discontinuity in the indicated pressure P_(IND).

As shown in FIG. 4 b, the indicated pressure P_(IND) may be adjustedover some range to gradually transition the difference in pressurereadings of two sensors when the processor is switching from a readingof one sensor to another sensor. In the exemplary embodiment in thetransition, the signal processor 455 may use the prior pressure readingsfrom what may be referred to for purposes of description as the“switching from”=sensor (e.g. the sensor the processor is switchingfrom) to adjust the current reading from the “switching to” sensor (e.g.the sensor the processor is switching to in order) to produce the outputsignal. FIG. 4 b shows, in one example, how the output signal P_(IND)may vary with respect to the true pressure in transition betweensensors. As may be seen, when true pressure is relatively high and isfalling, the indicated pressure P_(IND) may correspond directly to, andbe substantially equal to, the reading from heat loss sensor 440, andtherefore may follow line 502. When the pressure reading from sensor 440falls below threshold pressure P_(SW), P_(IND) then may becomedetermined by a reading from sensor 445. However, P_(IND) is adjustedrelative to the reading from sensor 445 (the “switching to” sensor) tocompensate for the differences in readings from the two sensors at thepoint where P_(IND) is switched from being determined by the sensor 440(the “switching from” sensor) reading to being determined by the sensor445 reading. Thus, P_(IND) follows along curve 504 as the true pressurecontinues to drop. At some point, curve 504 and line 501 meet, andP_(IND) is equal to the pressure reading of sensor 445. Below thispressure, P_(IND) may correspond directly to, and be substantially equalto, the reading of sensor 445. In the exemplary embodiment there may notbe any time at which P_(IND) is determined from the current readings ofboth sensors 435, 440, 445. Rather, the indicated pressure P_(IND) maybe determined from only one sensor reading throughout the cumulativepressure range of the instrument 400. FIG. 4 b also shows curve 505which may represent transition of P_(IND) in a rising pressureenvironment. When pressure is relatively low and rising, P_(IND) may bedetermined from and may be substantially equal to a pressure readingfrom sensor 445, until P_(IND) reaches P₃ and the processor switches forexample from sensor 445 to sensor 440). Then P_(IND) may followtransition curve 505 as the pressure continues to rise, and may bedetermined by a current pressure reading of sensor 440. While followingcurve 505, P_(IND) be an adjusted value relative the reading of sensor440. This may prevent an abrupt change in the value of P_(IND) whentransitioning from a state in which P_(IND) is determined from sensor445 to another state in which P_(IND) is determined from sensor 440. Asthe pressure continues to rise, curve 505 meets line 502, and P_(IND) isequal to a reading of sensor 440. For higher pressures, P_(IND) maycorrespond directly to, and may be substantially equal to, a pressurereading of sensor 440.

FIG. 5 is a flow chart that illustrates one exemplar of the processingperformed by the signal processor 455 in generating the indicatedpressure, such as of a chamber of apparatus 100, from the pressurereadings of the instrument sensors 435, 440, 445. Example resultsproduced by operating the signal processor 455 according to the flowchart are shown in FIG. 6. In the exemplary embodiment illustrated inFIG. 5, blocks 600 and 605, labeled PS1 and PS2, may represent thepressure reading PS1 (described before, see also FIG. 6) from one of thesensors, such as from the heat loss pressure sensor 440. Pressurereading PS2 (also described before) may be a pressure reading fromanother of the sensors, such as from the piezo-resistive diaphragmpressure sensor 435. Similarly, PS2 and PS1 may be the respectivereadings from the HL and IG sensors 440, 445. In block 6001, the systemmay be initialized. As may be realized, system initialization may beperformed at any time such as at startup/booting of the processor 455 ofinstrument 400. For example, the processor may perform a desiredinitialization routine that may confirm proper operation of theinstruments components (including sensors 435, 440, 445). The processormay select the operating parameters such as the respective pressureranges to be utilized by the processor for the corresponding sensors andthe transition points (i.e. P_(sw)) where the processor is to switchsensor readings, as well as the program algorithm (or table) to be usedto adjust the valve of the corresponding sensor when transitioningbetween sensors. The processor 455 may select operating parameters forexample by registering the gas species block 600A in the chamber (asnoted before sensor pressure ranges, and hence transition pressures mayvary with gas species). The gas species (e.g. air, N2, argon) may beregistered for example by operator input via the I/O device 510 (seeFIG. 3) or any other system or device capable of communicating the gasspecies to the processor. With the gas species registered, the processormay select in block 600B corresponding pressure range of each sensor435, 440, 445 for the registered gas species. The processor may effectselection for example by accessing a look-up table from internal memory456 correlating pressure ranges for the sensors with gas species. Asnoted before, the processor may for example access informationcorrelating or establishing pressure ranges of the sensors for theregistered gas specie from remote sensors. In alternate embodiments, thepressure range of each sensor 435, 440, 445 may be selected by theoperator entering the respective range for each sensor via the I/Odevice or any other selector. The transition pressures P_(sw) may bedefined by the selection of the pressure ranges for the sensors. Forexample, as shown in FIGS. 4 a, 4 b the transition pressure P_(sw) maybe set, when the sensor pressure ranges are determined as describedpreviously, to correspond to the ends of the sensor pressure range (e.g.P_(sw) may be set equal to the indicated pressure at the ends, Pc, PD ofthe sensors pressure range, respectively curved 502, 501). Thetransition pressures P_(sw) values may be preset and stored for examplein the processor memory 456, or in alternate embodiments in any memorylocation accessible by the processor, so that upon selection of thepressure ranges of the sensors the value of P_(sw) corresponding to theselection becomes identifiable by the processor. In other words theprocessor is given the value of P_(sw) for the pressure range selection.In alternate embodiments, the processor may be programmed to determinethe value of P_(sw) in order to take advantage of the best data from thesensors. As noted before, the accuracy and ranges of different sensors(of the same type) may vary (see for example FIG. 6). Hence, somesensors may have larger ranges, for a given species, than other sensorsof the same type. This is illustrated in FIG. 4 a by extension line 502a, terminating at end pressure Pc¹, showing an increased range comparedto the range of the sensor indicated by line 502. In view of theincreased operating range of the sensor, it may be desired to establishthe transition pressure identified in FIG. 4 a as P_(sw)′ to takeadvantage of the larger range of the sensor and hence set the value ofP_(sw)′ lower compared to P_(sw) (of a sensor with a smaller pressurerange). By way of example, the sensor with the larger range (relative toothers of the type) may have some advantage over the one or more of theother sensors 435, 440, 445 of the instrument (e.g. faster response,greater accuracy, reading substantially species independent) and henceit may be desired to employ this sensor over its larger range. By way ofexample, the PRD sensor 435 may have a quicker response than the HLsensor. Accordingly, it may be desired to use the PRD sensor to thelargest extent possible and hence set the switching pressure Psw′ (forswitching between the PRD and HL sensor) substantially at the limit/endPc′ of the PRD sensor. Hence, the processor 455 may be programmed withthe aforementioned selection criteria or factor, setting the value ofthe switching pressure Psw, Psw′, such as between PRD and HL sensors tothe lower end of the PRD sensor range, and adjusting the identifiedpressure range of the HL sensor to end at the pressure Pd′ correspondingto the set switching pressure Psw′ as noted before, the processor 455may be programmed with or may be capable of accessing the data forcalculating the value Psw, Psw′ (e.g. from the identified end points Pc,Pc′ and sensor calibration value) and similarly for adjusting thepressure range of the HL sensor. The processor 455 may be programmedwith other desired factors for selecting the switching pressure Pswbetween sensors such as setting the value of the switching pressure tocorrespond to the sensor with higher accuracy or any other desiredselection factors.

Each of the absolute readings PS1, PS2 serve as inputs via block 620. Inaddition, in block 610 a startup mode is initially set. In block 610, amode is set to PS2−. The mode is an indication of which sensor readingis currently being used by the processor 455 to determine the indictedpressure P_(IND), as well as whether the pressure is rising or falling.In this example, the pressure is initially high and falling, thereforethe mode is set to PS2− to indicate that the P_(IND) should be initiallydetermined from PS2 (i.e. based for example on the PRD sensor ratherthan the heat loss or ionization sensors), and that the pressure isfalling. Also in block 610, the processor may also access the selectedtransition parameters and set the value of variable KS1 equal to 1 andvariable KS2 equal to 0. These variables may be used in connection withthe transition parameters to determine P_(IND) in other blocks asdescribed below. In block 625, a determination is made as to whether thePS1 reading is valid (e.g. within the sensor pressure range). In thisexample, the PS1 reading is determined as valid if it is less than apredetermined value P_(HH). P_(HH) may be an upper endpoint of anoperating range of the heat loss sensor 440, above which the heat losssensor reading PS1 may be insufficiently accurate. In block 630, thecurrent mode is read back to determine whether the pressure is rising orfalling. If falling, block 635 is executed. In block 635, PS2 iscompared to selected value P_(SW). If PS2 is less than P_(SW), block 640is executed. Otherwise block 690, described below, is executed. In block640, the mode is set to PS1− to indicate that P_(IND) is currentlyresponsive to the PS1 reading from the heat loss sensor. In block 645,PS1 is compared to threshold value P_(L), which is the value of an upperendpoint of a region wherein P_(IND) directly corresponds to the readingPS1 from the heat loss sensor. If PS1 is less than P_(L), block 650 isexecuted. Otherwise block 705 is executed to produce a value forP_(IND), as described below. In block 650, PS1 is compared to thresholdP_(LL), which may be a lower endpoint of an operating pressure range forthe PDR sensor 435, below which the PRD sensor may not be operable toproduce a sufficiently accurate reading. If PS1 is less than P_(LL),block 655 is executed. Otherwise, block 665 is executed. In block 665,the mode is set to PS1+ to indicate that P_(IND) is currently responsiveto pressure reading PS1 from heat loss sensor 440 and pressure isfalling. In block 660, KS2 is set to PS2−PS1. In other words, KS2 is setto the difference between the pressure reading generated from the PRDsensor 435 and the pressure reading from the heat loss sensor 440. Instep 665, P_(IND) is set equal to PS1, the reading from the heat losssensor 440, and block 620 is re-executed.

If at block 630 the mode is determined to be positive, indicating arising pressure, block 670 is executed. In block 670, PS1 is compared toP_(SW). If P_(SW) is less than PS1, block 675 is executed. Otherwise,block 655 is executed. In block 675, the mode is set to PS2+ indicatingthat the indicated pressure P_(IND) is responsive to pressure readingPS2 generated with the PRD sensor 435, and that the pressure is rising.In block 680, PS2 is compared to P_(H), which may be an endpoint of apressure region within which the indicated pressure P_(IND) correspondsdirectly to pressure reading PS2 generated with the PRD sensor 435. IfPS2 is greater than P_(H), block 685 is executed. Otherwise, block 710is executed to generate a value for P_(IND). In block 685, PS2 iscompared to P_(HH), which may be an upper endpoint of a range of heatloss sensor 440 above which the sensor 440 is may not be sufficientlyaccurate. If PS2 is greater than P_(HH), block 690 is executed.Otherwise, block 700 is executed. In block 690, KS1 is set equal to theratio of PS2 over PS1. In block 695, the mode is set to PS2− to indicatethat P_(IND) is responsive to PS2 and is falling. In block 700, P_(IND)is set equal to PS2. Block 620 is then re-executed.

Block 705 is executed when the pressure is falling, reading PS2generated from the PRD sensor 435 is below threshold value P_(SW), andreading PS1 from the heat loss sensor 440 is above threshold valueP_(L). This is the range wherein P_(IND) is responsive to PS1 and notresponsive to PS2, and where P_(IND) is an adjusted value of PS1 withthe adjustment smoothing out the P_(IND) curve and avoidingdiscontinuities, jumps, and high-magnitude differentials. Block 705defines a value for P_(IND) according to the following formula:

$P_{IND} = {{PS}\; 1\frac{1 - {P_{L}\frac{{{KS}\; 1} - 1}{P_{SW} - P_{L}}}}{1 - {{PS}\; 1\frac{{{KS}\; 1} - 1}{P_{SW} - P_{L}}}}}$

Block 710 is executed when the pressure is rising, reading PS1 generatedfrom the heat loss sensor 440 is above threshold value P_(SW), andreading PS2 generated from the PRD sensor 435 is below threshold valueP_(H). This is the range wherein P_(IND) is responsive to PS2 and notresponsive to PS1, and where P_(IND) is an adjusted value of PS2 withthe adjustment smoothing out the P_(IND) curve and avoidingdiscontinuities, jumps, and high-magnitude differentials. Block 710defines a value for P_(IND) according to the following formula:

$P_{IND} = \frac{{{Ps}\; 2} + \frac{P_{H} \times {KS}\; 2}{P_{SW} - P_{H}}}{1 + \frac{{KS}\; 2}{P_{SW} - P_{H}}}$

Referring again to FIG. 6, there is also shown an example diagram ofvalues for F_(IND) that may result from the process illustrated by theflow chart of FIG. 5. As may be seen, there is no discontinuity in theindicated value P_(IND) at the threshold value P_(SW), regardless ofwhether pressure is rising or falling. Because there are no abruptchanges in P_(IND) as would occur if P if P_(IND) were simply switchedbetween P_(IND)=PS2 and P_(IND)=PS1 at the threshold pressure P_(SW),calculating a derivative or slope of P_(IND) at or near threshold valueP_(SW) (or elsewhere on the P_(IND) curve) may not result in a value ofhigh magnitude. Therefore it may be possible to use a derivative ofP_(IND) as feedback to control pressure, such as for example pressure ofload lock atmosphere 450.

As noted above, the exemplary measurement instrument may have anionization sensor 445 in addition to the PDR sensor 435 and heat losssensor 440. Readings from all three of the sensors may be used inproducing an indicated pressure output operable over an extended range.For example, at pressures below an operating pressure range of the heatloss sensor 440, the indicated pressure may be determined from a readingof ionization sensor 445, which may operate at a lower pressure rangethan sensor 440. The transition between a state wherein the indicatedpressure is determined from the reading of sensor 440 to a state whereinthe indicated pressure is determined from the reading of sensor 445 maybe handled in substantially similar manner as described previously, withreference to FIGS. 5 and 6, for the transition between using a readingof sensor 435 and using a reading of sensor 440. Thus, the measurementinstrument may be operable to indicate pressure across an extendedoperating range that is inclusive of an operating range of ionizationsensor 445 as well as operating ranges of heat loss sensor 440 and PRDsensor 435. Throughout the extended operating range of the measurementinstrument 400, an indicated pressure may be always determined from areading of only one of the sensors, yet the pressure may be indicatedwithout any jumps or discontinuities in indicated pressure created bytransitioning from one sensor reading to another. In other embodiments,the measurement instrument may have, for example, two sensors, or mayhave any other suitable number of sensors. In other embodiments, anysuitable physical characteristic may be measured by the sensors andindicated by the instrument (e.g. mass, force, light intensity, magneticfield strength, or other physical quantity or characteristic).

It should be understood that the foregoing description is onlyillustrative of the invention. Various alternatives and modificationscan be devised by those skilled in the art without departing from theinvention. Accordingly, the present invention is intended to embrace allsuch alternatives, modifications and variances which fall within thescope of the appended claims.

What is claimed is:
 1. An apparatus comprising: a housing; and a multi-sensor disposed within the housing, the multi-sensor including at least two sensors each being configured to generate a respective measurement signal of a physical characteristic over a respective one of a first measurement range and a second measurement range, and a processor connected to the at least two sensors and defining the first and second measurement ranges so that the first and second measurement ranges meet at a predetermined transition where a measurement embodied in the respective measurement signals transitions between the first and second measurement ranges without an abrupt change, the respective measurement signals being different at the predetermined transition.
 2. The apparatus of claim 1, wherein the predetermined transition for a rising physical characteristic occurs at a higher measurement value than the predetermined transition for a falling physical characteristic.
 3. The apparatus of claim 1, wherein the measurement embodied in the respective measurement signal for the first measurement range is adjusted relative to the measurement embodiment in the respective measurement signal for the second measurement range, over at least a portion of the second measurement range, so that a measurement indicated by the apparatus does not change abruptly when crossing the predetermined transition from the first measurement range to the second measurement range.
 4. The apparatus of claim 1, wherein the physical characteristic is a pressure.
 5. The apparatus of claim 1, wherein the physical characteristic is a gas density.
 6. The apparatus of claim 1, wherein the measurement embodied in the respective measurement signal for a first sensor of the at least two sensors is substantially equal to a measurement of the first sensor over at least a portion of the first range and the measurement embodied in the respective measurement signal for a second sensor of the at least two sensors is substantially equal to a measurement of the second sensor over at least a portion of the second range.
 7. The apparatus of claim 1, wherein one of the at least two sensors is selected from at least one of a piezo-resistive diaphragm sensor, a heat loss sensor and an ionization sensor.
 8. An apparatus comprising: a housing; and a multi-sensor disposed within the housing, the multi-sensor including at least two sensors each being configured to generate a respective measurement signal of a physical characteristic over a respective one of a first measurement range and a second measurement range, and a pressure indicator connected to the at least two sensors, the pressure indictor including a processor configured to provide an indication of a measurement embodied in the respective measurement signals that transitions between the first and second measurement ranges as a substantially continuous function over the first and second measurement ranges, the respective measurement signals being different at the transition.
 9. The apparatus of claim 8, wherein the transition for a rising physical characteristic occurs at a higher measurement value than the transition for a falling physical characteristic.
 10. The apparatus of claim 8, wherein the processor is configured to determine the physical characteristic from but one of the at least two sensors throughout a combined measurement range of the at least two sensors.
 11. The apparatus of claim 8, wherein a first sensor of the at least two sensors is configured to provide a first pressure reading and the pressure indicator is configured to indicate a pressure that is substantially equal to the first pressure reading when the first pressure reading is above a selectable predetermined pressure value.
 12. The apparatus of claim 11, wherein a second sensor of the at least two sensors is configured to provide a second pressure reading and the pressure indicator is configured to indicate a pressure that is substantially equal to the second pressure reading when the first pressure reading is less than a second predetermined pressure value lower than the selectable predetermined pressure value.
 13. The apparatus of claim 12, wherein the second predetermined pressure value is selectable from a number of different predetermined pressure values, and wherein the pressure indicator has a selector for selecting the selectable predetermined pressure value and the second predetermined pressure value.
 14. The apparatus of claim 12, wherein the pressure indicator is configured to indicate pressure as a continuously differentiable function over a range that includes a threshold for the second predetermined pressure value.
 15. The apparatus of claim 14, wherein the indicated pressure is without an inflection point between the selectable predetermined pressure value and the second predetermined pressure value.
 16. A method comprising: producing a first and second reading of a quantifiable physical property with a respective one of a first and second sensors; selecting, with a processor connected to the first and second sensors, a transition magnitude between a first range corresponding to the first reading and a second range corresponding to the second reading, where the transition magnitude of a rising quantifiable physical characteristic occurs at a higher magnitude of the quantifiable physical property than the transition for a falling physical characteristic; and adjusting the magnitude of the quantifiable physical property in at least one of the first and second ranges so an indication of the quantifiable physical property transitions between the first and second ranges without an abrupt change where the first and second readings are different at the transition.
 17. The method of claim 16, further comprising indicating a magnitude of the quantifiable physical property with the processor such that the indicated magnitude is adjusted only in the second range when the quantifiable physical property is falling.
 18. The method of claim 16, further comprising indicating a magnitude of the quantifiable physical property with the processor such that the indicated magnitude is adjusted only in the first range when the quantifiable physical property is rising. 